Metal oxide gels, such as silica gels, can be prepared in separate and distinct forms. The more common form is the hydrogel in which the voids in the metal oxide structure are filled with water. The other form is characterized as an aerogel. Aerogels are prepared from hydrogels by special techniques in which the water originally present in a hydrogel is displaced by a gas, usually air.
The best known method for preparing aerogels is that disclosed by Kistler in U.S. Pat. No. 2,249,767. Such aerogels have very low bulk densities and are useful for many industrial purposes.